1. Field of the Invention
The invention relates to a magnet system of an installation for nuclear spin tomography with at least one pair of annular superconducting coils for generating a homogenous magnetic background field. At least the superconducting coils located at the two front faces of the magnet system are each electrically bridged with a protective circuit to guard against an unintentional transition to a nonsuperconducting state i.e., a quench. The nonsuperconducting coils form magnetic field gradients. A hollow cylindrical thermal shield of a nonsuperconducting material is arranged between the gradient coils and the superconducting coils and extends in the axial direction at least up to the two front faces.
2. Description of Related Art
In the field of medical technology, diagnostic methods that use image producing have been developed to particularly analyze a human body. Integral resonance signals of the nuclei of particular chemical elements of the body or a part of a body are analyzed through calculations or measuring techniques. An image formed from the distribution of spatial spin densities or relaxation times is similar to an x-ray tomograph obtained in computer tomography and can be reconstructed and calculated, respectively. Corresponding methods are known by the name "nuclear spin tomography," Nuclear Magnetic Resonance tomography or (NMR) tomography, also referred to as "Magnetic Resonance Imaging" (MRI) or "Magnetic Resonance Spectroscopy" (MRS) or "zeugmatography".
A precondition for nuclear spin tomography is a magnetic field generated by a background field magnet. This background field must be sufficiently homogenous within an examination or imaging region. The magnetic induction in this region can be several Tesla. Such magnetic inductions, however, can be economically generated only with superconducting magnets. A body or a part of a body is introduced into the examination area along an axis that generally coincides with the axis of orientation of the background magnetic field. This background field is superimosed by stationary or pulsed gradient fields. In addition, a special antenna device is required to excite the discrete atomic nuclei in the body into a precessional motion. The antenna briefly activates a high-frequency alternating magnetic field. If necessary, this antenna device can also act as a receiver for the high-frequency signals thus produced.
Larger superconducting magnets are used in, for example, installations for nuclear spin tomography. These larger magnets can store considerable amounts of energy, often in the MJ range. These magnets are particularly vulnerable during an unintentional transition from the superconducting operating state to a nonsuperconducting state. This transition is referred to as "quench" and initially occurs only in part of the magnet. The increased resistance and the low heat capacity of the superconducting coil conductors of the magnet following a quench cause these coils to heat rapidly. The specific resistance simultaneously increases which further accelerates the heating. The consequences are electrical overvoltages that stress the insulation and, in the event of a flashover, can lead to damage or destruction of the magnet.
Special measures are required to protect such superconducting magnets against damage or destruction through overheating and electrical flashovers. These measures can comprise, for example, subdividing the magnet into several discrete coils. These coils are additionally electrically bridged with their own protection circuit to further limit the voltage. Such protection circuits include ohmic protective resistors (cf. German Pat. No. 2,301,152), semiconductor diodes (cf. German Pat. No. 1,614,964) or arresters (cf. German Pat. No. 1,764,369).
The superconducting background field magnets that are used in a nuclear spin tomography installation can be composed of several pairs of discrete annular superconducting coils and thus have the form of a solenoid (c.f., for example, European Patent Application No. 105,565 or German patent application No. 3,344,047). These discrete superconducting coils advantageously can be electrically bridged with the quench protection circuits. In the event of a quench of a single coil, however, the currents of the coils of the magnet, generally connected in series, can show a strongly differing shape over time so that the otherwise symmetrical current and, consequently, also the field distribution become strongly asymmetrical. The same thing also happens in an interaction of the coils with their surroundings. Undesirable circular currents are induced in the walls surrounding a cryostat and particularly in the surrounding radiation and thermal shields. The radiation shields that face the examination area are preferably made of thick walled pipes comprising a good electrical conductor so as to obtain a high electromagnetic time constant and a corresponding reduction in the effects of eddy currents on the gradient fields. However, high quench-induced currents can produce a sum of axial forces in the range of several tons by acting together with the coil field. This force stresses the mechanical suspension or support of the superconducting coils. The greatest forces are generated in a quench of one of the front face coils of a multicoil system of the type often provided for nuclear spin tomography background field magnets.